Hypoxic cells are generally three-fold more resistant to ionizing radiation-induced cell kill than are fully oxygenated cells. Since there is evidence that human tumors contain hypoxic cells, it is possible that those cells limit the curability of some human tumors by radiotherapy. Improvement of radiotherapy treatments of cancer might be realized if the problem of hypoxic cells could be eliminated. Therefore, it is critical that the mechanisms by which hypoxic cells are made radioresistant or, conversely, the mechanisms by which oxic cells become sensitized (the oxygen effect) be understood. The oxygen effect has been the topic of many previous studies; a great deal of information has been obtained regarding oxygen's modes of action, but many questions remain. In order to understand the mechanisms of the oxygen effect, a radiation chemical approach to the question should be taken. In this project bacterial transforming DNA and plasmid DNA systems will be used. Purified DNA will be irradiated under carefully controlled conditions, such that the radiation chemical reactions occurring in the solutions are known or can be predicted, yet the DNA damage will be assessed by studying the biological functioning and single-stranded integrity of the DNA. This approach makes it possible to elucidate the chemical reactions involved in radiation-induced DNA damage and modification of that damage by oxygen. This information will aid in the design of methods to effectively overcome the problem of hypoxic cells in human tumors. The proposed project will address five specific questions: (1) How does the high local concentration of DNA in cells alter radiosensitivity and modification of that sensitivity by oxygen? (2) Do membrane components, lipids and proteins, radiochemically interact with DNA in such a manner that oxygen modification of damage is seen, and does this support the hypothesis that the DNA-membrane attachment site is the target for the oxygen effect? (3) Does the close association of cellular DNA with polyamines and histones alter DNA radiosensitivity and modification thereof by oxygen? (4) Is the oxygen effect dependent upon interactions with sulfhydryl-containing compounds? (5) Are radiation-induced DNA strand breaks involved in or critical for the loss of biological function of DNA, and how does the presence of oxygen modify that contribution?